Toss the fries with 1/8 teaspoon five-spice powder and 1/2 teaspoon salt on a baking sheet; bake as the label directs until crisp.
Meanwhile, preheat a grill pan over high heat. Mix the ketchup, relish, Sriracha and the remaining 1/8 teaspoon five-spice powder in a bowl; set aside 2 tablespoons for brushing the steak.
Prick the steak all over with a fork; season with salt. Brush the grill pan with oil, then add the steak and grill until marked, 12 to 15 minutes, flipping and brushing occasionally with the reserved spiced ketchup. Transfer to a cutting board; let rest 10 minutes, then thinly slice against the grain.
Toss the cucumber, vegetable oil and vinegar in a bowl; season with salt and pepper. Toss the fries with the scallions, and salt to taste. Serve with the steak and the spiced ketchup.
Photograph by Christopher Testani